This invention lies in the field of smokeless combustion of waste gases in flare stacks. More particularly it concerns method and apparatus for utilizing low pressure steam as a smoke suppressant in the burning of waste gases. Smoke results because of the presence of free carbon as it escapes from the combustion zone. A result of injecting water or steam into the heated zone of burning hydrocarbons is based on typical reforming chemistry equation: EQU CH.sub.4 +H.sub.2 O=CO+3H.sub.2
especially where the water is in vapor phase. One factor in suppression of smoke is through the reaction as shown, where carbon is combined with oxygen to form carbon monoxide, which is both invisible and rapid burning. Significantly, and perhaps more important are other factors such as inspiration of air caused by the steam injection plus the resultant turbulence and time of reaction to supress smoke.
Many refineries and gas process plants have low pressure steam (e.g. 10 to 50 psig) available. Typically, such steam is normally exhausted to the atmosphere. Energy costs now demand conservation and use of such steam. Heretofore low pressure steam was rarely used as a smoke suppressant in flares unless special equipment was created to pre-mix the steam and waste gas, as typically shown in U.S. Pat. Nos. 3,973,899; 4,152,108. Other background patents are U.S. Pat. Nos. 3,749,546 and 3,887,324.
A problem exists in the use of low pressure steam where it is injected as a function of demand for smoke suppressant. That is, high demand requires high steam flow and low demand a lesser steam flow. In those instances, where steam flow is at a maximum rate for a given design, the corresponding pressure drop across the injector (e.g. 30 p.s.i.g. available steam source) is such that the amount of energy available will promote smokeless burning. But where the demand for steam becomes less, the pressure drop across the injector is less and is thus ineffective to suppress smoke.
Another factor in the smoke suppressant art is the design of the steam injection orifices or nozzles so as to make maximum use of the available steam pressure. The pressure of the steam is directly relative to its potential mechanical kinetic energy and hence the penetrafility of steam and inspirated air into the burning gases for smokeless burning. The most difficult point of operation is at or below of the mechanical kinetic energy of a given orifice's sonic exit velocity. Below this point of operation efficiency of smoke suppression becomes very poor.